1. Field of the Invention
The present invention relates in general to a signal suppression apparatus and, in particular, to a vocal frequency suppression apparatus for suppressing any vocal frequencies which correspond to frequencies found within DTMF codes, so as to prevent the inadvertent activation of DTMF controlled components, such as voice mail systems.
2. Background Art
The problem of inadvertent activation of dual tone multi-frequency controlled components, such as voice mail systems, is well known in the art. Dual tone multi-frequency ("DTMF") codes are each comprised of low frequency components (697Hz, 770Hz, 852Hz and 941Hz) and high frequency component (1209Hz, 1336Hz, 1477Hz and 1633Hz) of equal magnitude. This scheme itself was developed to prevent inadvertent control of DTMF controlled devices. However, it has been found that some individuals inadvertently produce frequencies within their voices which correspond to various ones of the high and/or low frequencies of the DTMF codes-thereby inadvertently actuating some DTMF controlled components, such as voice mail systems.
Various approaches have been devised to prevent such occurrences. For instance, U.S. Pat. No. 2,620,398 to De Vries discloses direction-sensitive signal suppressors located at the "ends" of a transmission line and before any user's individual telephone line. These suppressors influence the value of vocal frequencies which correspond to DTMF codes so as to reduce the likelihood of inadvertently triggering signal receivers. De Vries utilizes an active filtering approach which locates "true" signals by comparing the magnitude of the high and low frequencies and attenuating "false" signals such that only the true signals proceed onto the transmission line.
U.S. Pat. No. 3,715,518 to Campbell et al. discloses a system for preventing inadvertent activation of a voice mail system by recorded DTMF tones. Campbell momentarily interrupts the outgoing recorded transmission when DTMF signals are detected to allow the detector to accept valid DTMF codes. If the tone stops (presumably due to the interruption of the recording) then the signal is assumed to be invalid.
U.S. Pat. No. 3,944,753 to Proctor et al. teaches an apparatus which detects voice and other noise signals which may be present on a telephone line utilizing three different detection systems concurrently. The first detection system is comprised of two frequency drop detectors. DTMF signals maintain a substantially constant frequency after their initial frequency rise and further maintain a relatively constant amplitude. Therefore, one can use a frequency drop detector to record rapid voltage drops of approximately 0.6V, a condition which is apparently impossible for a "true" DTMF signal. The second detection system compares signals to a DC threshold representing some frequency higher than 1633Hz, the highest standard DTMF frequency. The third detection system detects noise signals, such as those caused by a high amplitude whistle, which would pass through both band pass filters in Proctor at the same frequency. A frequency coincidence detector analyzes the frequencies in both the upper and lower band for identical frequencies, the location of which would indicate a noise signal. These three detectors can each independently suppress the input signal, such that it is not received by the DTMF tone receiver.
U.S. Pat. No. 4,042,790 to Richards cross compares the average voltage peak amplitudes of the signals in the upper and lower frequency bands of DTMF codes and where the average voltage peak amplitudes in one band exceeds the other by a ratio greater than approximately 3:1, the tone receiver output is inhibited. Richards also detects the average voltage peak amplitude in the frequency spectrum above 1700Hz, which is above DTMF signal range and compares this amplitude with the amplitudes of the upper and lower bands to determine, if the signal component above 1700Hz is greater than 0.25 of the upper or lower band amplitudes, the tone receiver output is again inhibited.
U.S. Pat. No. 4,747,126 to Hood et al. discloses an overall voice mail system in which DTMF command signals may be recorded and then at playback may inadvertently activate a command. After discounting the possibility of filtering tones from the tape record head input, because of the number and wide frequency range of the component tones, Hood provided a system which would record tones of a shorter duration than the duration required to activate the circuit during playback.
U.S. Pat. No. 4,885,763 to O'Brien et al. discloses a voice mail system which includes a method and apparatus for muting a previously recorded audio signal upon detection of a DTMF signal to differentiate between recorded versus potentially valid DTMF codes. U.S. Pat. No. 2,572,074 to Terry et al. discloses a guard circuit which blocks the operation of a tone receiver if frequencies other than the signaling (DTMF) frequency are present on the line. This is accomplished by using compensating circuits to offset the rise in voltage when more than one signaling frequency is received.
One shortcoming of some of the forementioned devices is that they are wholly integrated into their respective systems. Thus, requiring the owner of a system subject to inadvertent control problems to purchase an entire system to circumvent that problem.
Another shortcoming of some of the prior art devices, is the complicated and expensive approaches used to prevent activation.
A further shortcoming of some of the prior art, is their failure to provide protection against vocal frequencies which may "talk-off" a device.
It is thus an object of the present invention to provide a device for suppressing one or more vocal frequencies which correspond to DTMF frequencies from a vocal signal before that vocal signal is combined with generated DTMF codes so as to prevent inadvertent activation of DTMF controlled devices, such as voice mail systems.
It is an associated object of the present invention to provide an inexpensive, portable device which can be easily moved between systems to prevent inadvertent control of DTMF controlled devices.
It is an additional associated object to provide a simple, and thus relatively inexpensive device for suppressing the vocal frequencies corresponding to DTMF frequencies by requiring suppression of vocal frequencies corresponding to DTMF frequencies to occur prior to the introduction of generated DTMF codes.
These and other objects of the present invention will become apparent in light of the present specification, claims and drawings.